Weston 686 TYPE 10A User manual

INSTRUCTIONS

FOR

II WESTON

MODEL686 -TYPElOA

.True Mutual Conductance

Vacuum Tube Analyzer

WESTON ELECTRICALINSTRUMENT CORPORATION

NEWARK5, NEW JERSEY,U. S. A.

'.2551 3.54 Pri"ted in U.S.A.

1:1

Ii!!

INSTRUCTIONS

FOR

WESTON

MODEL 686 - TYPE lOA

True Mutual Conductance Vacuum

Tube Analyzer

CONTENTS PAGE

Controls, Variable

Control Grid Voltage AdjusteL_mmm m_umm_m4

Heater Voltage Adjusterm_uhm_mmum_m__uu__m___A

Hum mmmmm_u__u__um___m__umm 5, 7 (Step27)

Plate Voltage Adjuster:

See "D-C Power Supply Controls"mm u m_5

Screen Voltage Adjuster:

See "D-C Power Supply Controls"m_mmm__m_m5

Self- Bias Resistance 00 ___ _ 00 _00 __ __ m __ _ __ __ _ _ _ _00 __ _m___5

Signal Amplitude AdjusteL_m__uuuum_5, 8 (Step 29)

Suppressor Voltage Adjuster:-

See "D-C Power Supply Controls"m__m__m__mu__5

D-C Power Supply Controls_m_m_mmmmm m__5

Detailed Description of Model 686 Type 10A__3-4-5-6

Front Panel View of Model 686 Type 10Am_mmmm__2

General Description m m___n__m_m mm_3

General Information __nm__mm__h__mum_mm_mmmm_ 10

Gm Readings

Amplifier Tubes __hu mm___mm__u__mnm__mm_m7-8

Inconsistent on Filament Types:

See "Hum Control" _mm___mmm_mmu5, 7 (Step 27)

Low Gm Indications_muum__mumumm___m_m_hu 11

No G.m Indications_mm_m_mmmmm__mmmummuu 11

Grid Current _mmmm m_m_mm_m3, 7 (Step 28), 10

Helpful Suggestions in Tube Testinguhuum__h_uhum_1 0

Instruction Book um_mm_m__m m_m m h 10

Maintenance uhm_m m_m_m_mum_m_h__mmum 11

Meters

Control Grid Voltageu n__n mn_m__n nu4

Element Current _m m_m__m___m__mm m4

Element Voltage u umu__m mm 4

Gm (Mutual Conductance} m uu m___3

Grid Current __ 00__m__ um m__m ________00_3

Heater Voltage mmmmm_m_mmm m_mm__mm_h__4

Operation

Line Connections for 230 Voltsm___un__m__m_m m_6

Step-By Step Test Procedure__um__mm_un_m6-7-8-9

Theory of Operation_m_mm_m mumumm_ 9-10

CONTENTS PAGE

Order of Adjustment of Element Potentiometersmmm_5

Ordering Information _u__mm__mm mmm__m__m 11

Precautions __u__h__mm_mm_m_h mm_mm_umm m 11

Switches, Rotary

"Control Grid" (bias} mmm m_m___m_m_-4

"Heater Voltage" u__m__mm__mm_m_nnm__m_mm_mA

"Gm Ra nge" _00 0000000000 3

"Element Current" mm mnm__m_m_mmmm__m_m__A

"Element Current Range"m__m__mmmmmmmm_m_A

"Element Voltage" ummm_mm mm_m_m_m__mu4

"Element Voltage Rang,e" __mmmmm hum_m___mA

Switches, Toggle

"Gm Reading" __nm um m_m m__m 6

"H eater Supply" 00 0000 0000___000000 6

"Gm Factor" n nn ~ 3

"Lamp Test' 00 00 000000___ ___00 000000 6

"Microamperes" u__m_uum m__mm_m__umm 3

"Plate 1-Plate 2" uu__mm_mmm__mn m__um___uu5

"Plate Su pply" nu m__m m m6

"Signal Cal." 00000000 00 000000 0000 00___6

"Supp" (Pos-Neg) mmm umm umm__6

Tube Manuals m_m_mmnmm__m_m__m mmmmm 10

Tube Reject Limits mm mum mu m__n m___m_ 10

Tube Testing

Ampl ifiers _____u__m_mu 00_m__ ___u_m__ umu_ _____um _6-7-8

Converters & Mixer Oscillatorsm_mm__m_h mm__8

Diode Detectors ___mnum_m___m m m_mm8

Rectifie rs _ _ ___ __ _ _ __ 00_ _ _ ___ _ __ _ __ _ _ ___ _ __ _ __ __ _ _ _ ___ 00 __ _ _00 __ _ 00 8

Thyratro n s ________________________________________________----- -- -- --- 8-9

Voltage Regulators hu__mm__mmuu nmm_mm__mn9 '

Wiring Diagram of Model 686 Type 1OAu__m_humm12

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Front Panel View of Model 686 -Type lOA

-2-

INSTRUCTIONSFOR MODEL 686- TYPE lOA

TRUE MUTUAL CONDUCTANCE VACUUM TUBE ANALYZER

GENERALDESCRIPTION

The Model 686 is a complete direct reading True

Mutual Conductance Vacuum Tube Analyzer de-

signed to operate from any 105 to 125 or 230 volt

50-60 cycle outlet. It has eight mutual conductance

ranges with full scale readings of 300, 600, 1500 and

3,000 micromhos and also 3,000, 6,000, 15,000 and

30,000 micromhos. Instruments are provided for ac-

curately measuring all electrode voltages and for

reading electrode currents including minute grid

currents.

Internal power supplies and a signal source provide

all necessary potentials to panel controls, wherein

adjustments can be made in accordance with meter

readings. Tube sockets for all commercial type re-

ceiving tubes are mounted on a removable socket

panel across the top front section of the equipment.

These in turn connect through short-test switches to

patch cord jacks which are marked with R.M.A. pin

numbers and are used with patch cords for any or

all electrode connections. Thus with complete con-

nector flexibility and complete voltage control, all

kinds of static characteristics can be plotted, in addi-

tion to the measurement of Gillunder any or all ap-

plied potential conditions.

DETAILEDDESCRIPTIONOF EQUIPMENT

THEG./IIMETER:The fan shaped instrument in the top

center of the main panel is the Gillmeter. The scale

is calibrated in two arcs reading zero to fifteen and

zero to thirty and sixty. This meter operates with

two multiplier switches marked "GM RANGE" and

"GM Factor".

The Gill Range switch is used for selecting the

300,600, 1500, or 3,000 micromhos range as required,

with corresponding grid signal voltages of 1.0, 0.5,

0.2, and 0.1 volts respectively. The GIllFactor switch

is used to extend the ranges by a factor of ten times

so that ranges of 3,000, 6,000, 15,000 and 30,000 are

also available.

On low mu tubes with low plate resistance, the

meter resistance is a reasonable part of the total tube

and tube tester plate circuit impedance; measure-

ments on these tubes would be in error unless .this

was taken into consideration. The compensation is

made automatically for all tubes having a plate

resistance above 250 ohms.

On high mu tubes the peak value of the grid signal

.voltage should be kept smaller than the dc grid bias

voltage to prevent direct grid conduction. On tubes

having sharply curved characteristics the grid signal

should be kept small to avoid second-order effects.

The low signal voltages available make it possible

to measure all low bias tubes operated and tested

as Class A amplifiers.

It is advisable for the operator to select the low-

est grid signal voltage that will render reasonable

pointer deflection when testing all low bias high mu

tubes.

Note that a 3,000 micromho range is available with

either a 1 volt signal or a 0.1 volt signal. Use the

0.1 volt signal for low bias high mu tubes. The 1 volt

signal is useful for tubes having a-c operated filia-

mentary cathodes, where it is desirable to increase

the signal voltage to minimize hum effects.

The a-c grid signal voltage is calibrated on the

Gill meter to eliminate possible errors due to temper-

ature and amplifier gain. If the instrument is reading

the a-c plate current component 2% low because of

these effects, it will likewise require a signal voltage

2% higher in value to bring the meter to top mark

on Signal Calibrate. Since the signal voltage is

higher, the GIll indication will also be higher and

hence compensation is obtained.

GRIDCURRENTMETER:A two range microammeter

for grid current readings is mounted to the left of the

Gill meter. This instrument has a range of 15-0-15

microamperes. Readings down to and including one-

half microampere are easily read. The meter is nor-

mally shunted to 1500-0-1500 microamperes and is

switched to the low range by manipulation of a mo-

mentary toggle switch located to the right of the

meter.

The meter is a zero center instrument to indicate

any or all components of grid current resulting from

gas, leakage resistance, or secondary emission. Grid

current readings are especially important in segre-

gating defective power tubes such as the type 6L6

where a limit of 3 microamperes is specified.

In taking grid current readings, the operator will

note that there is a red line on each side of the cen-

ter scale zero mark. This red line indicates 15micro-

amperes on the 1500 microampere range. If the

pointer does not deflect beyond the red line after the

tube is heated, then the Microamperes switch can be

shifted to the 15 microampere position, and readings

taken on this low range. The instrument is in series

with the control grid patch cord jack at all times and,

therefore, will indicate grid current under all operat-

ing conditions.

-3-

jELEMENTCURRENTMETER: Afour range d-c milliam-

meter used to measure cathode, suppressor, screen,

or plate current. The Element Current Range switch

below the instrument serves to select either the 3, 12,

60 or 120 milliampere range. The Element Current

switch directly to the left of the range switch selects

the electrode in which the current is to be measured.

;

The Element Current meter has less than 75 milli-

volts drop in the electrode circuit to which it is con-

nected. The switch is designed so that all other posi-

tions are short circuited. This switch may be indexed

to any position at any time without effecting read-

ings.

CONTROLGRIDVOLTMETER:

Measurements of grid

bias potential are made on the Control Grid Voltage

meter. The switch to the right of the meter selects

either the 10, 50 or 100 volt range as required. Be

sure to note the position of this switch when unex-

pected plate current exists in the tube under check.

This switch also changes the bias network, de-

creasing the drop across the control grid potentio-

meter on the 10 volt range, providing better voltage

control when adjusting low bias potentials.

Note that the grid bias supply is regulated by an

OB2 regulator tube.

LFIG. 1

HEATERVOLTMETER:Filament potentials from 0.6 to

120 volts may be selected by means of a rotary

switch and potentiometer. These potentials are meas-

ured on a multirange a-c voltmeter (Refer to Fig. 2).

Note that the Heater Voltage Selector switch is

used to select the nominal voltage, and exact adjust-

ment is made by rotating the Heater Voltage Adjuster

which controls the primary potential on the filament

transformer. An interlock circuit is used on the rotary

switch to automatically shift ranges on the filament

voltmeter as the switch is rotated. Thus the operator

always has this meter on the correct range, and the

possibility of instrument overload is eliminated. The

particular range connected into the circuit is marked

for each group of potentials on the Heater Voltage

Selector switch plate. The voltmeter connections are

brought back through separate leads directly from the

tube socket thus providing a more accurate reading of

filament potential at the tube pins. Thus any voltage

drop in the leads from the filament transformer to

the tube socket will not be in the meter circuit.

FIG. 2

ELEMENTVOLTMETER:

Electrode potentials are meas-

ured on the two range Element Voltage instrument

mounted below the Element Current meter. Plate,

screen, positive suppressor grid, or negative sup-

pressor grid potentials can be selected by using the

rotary switch below the meter (Refer to Fig. 3).

When selecting either Pos Supp or Neg Supp volts

position on the rotary Element Voltage switch, choose

the position indicated by the Supp-Pos-N eg toggle

switch located just to the left of the Model 686 name

plate. If the rotary switch is indexed to the wrong

position with respect to the positioning of the toggle

switch mentioned above, the Element Voltage meter

will read down scale. No damage will be done to

the voltmeter if a down scale reading should occur.

It must be remembered that when checking any

one tube, either a negative suppressor volts or a

positive suppressor volts (used for second anode

voltage purposes) can be selected, but not both for

the same tube test.

The Element Voltage Range switch directly to the

left of the meter selects the Element Voltage meter

range of 60, 150 or 300 volts. This switch is compen-

sated for meter loading so that the effect on validity

of readings is within 0.2%.

-4-

-------

FIG. 3

D-CPOWERSUPPLYCONTROLS:Individual suppressor

grid, screen grid, and plate adjusters are mounted

in line across the lower section of the Model 686

panel. These are 150 watt or 100 watt vitreous type

potentiometers connected in the d-c supply circuit.

These high wattage ratings are used to provide a

long peripheral length of contact travel, thus giving

accurate potential settings on the tube electrodes. The

element voltages should be adjusted in the following

order to prevent damage to the screen and cathode

due to excessive currents: (1) control grid (2) plate

(3) screen. All electrode potentials can then be re-

adjusted and any changes due to tube loading of

the power supply can be corrected. The accuracy

of the mutual conductance readings depend to a

great extent on the accuracy of these electrode poten-

tial adjustments.

SIGNALAMPLITUDEADJUSTER:

By effectively chang-

ing the signal voltage this potentiometer is used to

compensate for different line voltages. The signal

voltage is checked at full scale on the Gillmeter and

any necessary correction is then made with this

control.

To correct for different line voltages the Signal

switch should be placed on the "Ca1." position and

the Signal Amplitude Adjuster rotated to obtain full

scale deflection on the Gm meter.

HUM CONTROL:Filamentary tube types require an

accurate electrical center tap on the filament for

correct Gm readings, and it is the purpose of this

control to provide such an adjustment. On heater

voltage switch positions 12.5 volts and above, the

Hum Control is disconnected to prevent excessive

heat dissipation.

After the tube is in position and all electrode

potentials have been adjusted to the specified val-

ues, the GillFactor toggle switch to the right of the

Gillmeter should be set to the "x 1" position and the

Hum Control set for a minimum or zero reading on

the Gill meter. (Serious errors in mutual conductance

readings can be expected if this operation is not per-

formed carefullyJ

SELFBIAS RESISTANCECONTROL:The Model 686 is

equipped with a Self Bias Resistance control mounted

on the left side of the top panel section. This control

provides for checking certain tube types such as the

6I4, 6I6 and 1231 where self bias is definitely speci-

fied by the manufacturer. These tubes tend to draw

grid current or are unstable under equivalent fixed

bias conditions, thus causing errors in Gmreadings.

The control may be varied from 0 to 700 ohms,

and to prevent degeneration the unit is by-passed

by a 1,000 m.f., 50 volt condenser. When checking

the normal fixed bias types this control must be set

to zero (0) but on those types requiring self bias the

Control Grid Voltage Adjuster must be set in the

most counter-clockwise position. This procedure is

necessary to prevent fixed bias from being applied

in addition to the self bias.

To prevent damage to the 1,000 microfarad con-

denser, a simple calculation should be made to see

that the product of the expected cathode current,

and the resistance in the self bias circuit does not

exceed 50volts: V == 1x R x .001

Where V == Voltage appearing across 1,000

microfarad condenser.

I== Expected cathode current in mill-

amperes.

R ==Resistance selected by Self Bias

switch position.

SHORTTEST-METERREADINGAND ELEMENTSHORT

TESTSWITCHES:The Short Test-Meter Reading and

Element Short Test switches mounted on the right

side of the top panel section provide the necessary

means for short checking tubes with a d-c potential.

Tubes, having a filament or heater, may be short

checked, either hot or cold. The Short Test-Meter

Reading switch disconnects the d-c potential and

meters from the tube elements and connects a small

d-c power supply and a neon lamp into a group of

circuits controlled by the Element Short Test switch.

This switch segregates the element to be short

checked, leaving all the other elements tied together.

The filament or heaters should be at normal op-

erating temperature when hot short checking, and

the Short Test-Meter Reading switch should be in-

dexed to "Short Test" position. The Element Short

Test switch is then rotated through its six positions,

stopping at each position to tap the tube.

If the patch cords have been connected so as to

use both PI = Plate 1 and Pz =Plate 2, index the

Plate 1-Plate 2 toggle switch located on the upper

right top panel section to its other position and rotate

the Element Short Test switch through its six positions

again. A slight flicker of the neon lamp between

positions on the Element Short Test switch does not

indicate a short in the tube.

-5-

HEATERSUPPLYTOGGLESWITCH:Located at the bot-

tom left side of the panel with its associated green

jewel pilot lamp, this toggle switch disconnects the

device completely from the line supply when in-

dexed to the "Off" position. When in the "On" posi-

tion, the heater transformer is energized and the line

supply voltage is delivered to the Plate Supply

Toggle Switch. After making the heater connections,

the tube can be warming up while the operator is

completing the patching operation by indexing the

Heater Supply toggle switch to the "On" position.

The "Plate Supply" toggle is shorted out on 230 volt

service, therefore, use the "Line Switch" as the "On-

Off" switch for the entire device.

PLATESUPPLYTOGGLESWITCH:This switch operates

all of the internal d-c power sources and is inopera-

tive if the Heater Supply toggle is in the "Off" posi-

tion. The Plate Supply toggle should always be in

the "Off" position when the operator is changing or

removing patch cords.

SIGNALCALIBRATETOGGLESWITCH:To the right of

the Heater Supply toggle is a momentary type Signal

CaI. toggle switch. A change in power line voltage,

of course, will affect the grid signal and hence it

becomes necessary to check this potential just before

taking Gmreadings. Indexing the Signal Cal. toggle

to the "CaI." position places the grid signal directly

across the Gill meter. Regardless of Gm range se-

lected, the Gmmeter must indicate top mark on this

test. If it does not, the Signal Amplitude Adjuster

should be rotated until top mark is obtained.

Gill READING TOGGLESWITCH: This switch is located

to the left of the Plate Supply switch and is of the

momentary type. It is used to apply the signal volt-

age when the operator is ready to take a Gm read-

ing.

SUPPRESSORPOSNEGTOGGLESWITCH:The purpose

of this toggle is to allow the operator to select either

a negative potential for suppressor, or a positive

potential for tubes requiring a second anode voltage

such as the oscillator plate in a penta grid converter.

It must be kept in mind that while checking a given

tube only one or the other potential may be used.

No occasion will arise requiring both a negative

suppressor potential and a second anode positive

potential, because in such cases, the suppressor of

the tube is always connected directly to cathode.

To obtain negative suppressor volts index this

toggle, located just to the left of the name plate at

the bottom of the panel, to the NEG position. To

measure the negative potential rotate the Element

Volts switch to the "Neg Supp" position.

LAMPTESTTOGGLESWITCH:Located to the left of the

rotary Plate I -Plate 2 switch, this toggle provides

a ready means for checking the neon lamp. If a

tube shows no short and there is some doubt as to

the condition of the neon lamp, lift the toggle to its

upper position and if it does not glow replace with

a new lamp.

OPERATION

STEP-BY-STEPTESTING PROCEDURE

GENERAL:Read the paper tag on the f!ont panel of

the instrument. Remove the back cover and insert

the 5U4-G rectifier in the octal socket. Ascertain

whether the 3A4 tube is in place by removing the

shield from the miniature socket. Check to see that

the proper miniature tubes are in their respective

shielded sockets. Screw the neon lamp in the socket

located above the Lamp Test toggle switch. The line

cord can then be brought to the outside of the case

and the back cover replaced.

A toggle switch has been incorporated to facilitate

change over from 115 volt to 230 volt a-c operation.

This switch is located on the shelf jnside the device

between the 8 mfd. condenser and the small signal-

short check transformer.

The toggle is normally set for 115 volt operation.

For 230 volt operation set the switch as indicated

by adjacent markings.

The Plate Supply toggle is shorted out on 230volt

service, therefore, use the "Line Switch" as the "On-

Off" switch for the entire device.

The instrument is now ready for use and the

Step-by-Step Procedure outlined below should be

read carefully before attempting to check any tubes.

It is suggested that a type 6C4 or any indirectly

heated triode be used to acquaint the operator of

this device with the various controls and their

functions.

AMPLIFIERTUBES:

1. Plug the line cord into a power source having

a frequency of 50-60 cycles and voltage betwee.n

105-125.. Refer to preceding paragraph for 230 volt

lines.

2. Place Heater Supply and Plate Supply toggle

switches in "Off" position.

3. Patch the jumper leads in accordance with the

tube base diagram of the tube to be checked follow-

ing the specific procedures for Cathode, Heater and

Suppressor connections as follows:

A-CATHODE

I-Connect cathode of the tube to any of the

three Cathode jacks in the bottom row when

the Self Bias Resistance control is set to

"Fixed Bias."

II-Connect the tube cathode to the second or

third Cathode jack if Self Bias Resistance

control is set to any value above zero ohms.

-6-

B-HEATERS

I-Connect the heater or filament of the tube

to the Heater Supply jacks.

II-Duplicate the connections from the heater of

lhe tube and connect to the Heater Meter

jacks.

C-SUPPRESSOR

I-Connect suppressor to Suppressor Grid jack

except for the following two conditions.

(a) When other tube elements require a pos-

itive voltage from the Suppressor Grid

jack follow steps II or III outlined below.

(b) When Self Bias Resistance control is set

to or any value above zero ohms, step

III outlined below must be followed.

II-Connect suppressor of tube to any of the

three Cathode jacks in the bottom row when

the Self Bias Resistance control is set to

"Fixed Bias" or zero ohms.

III-Connect suppressor of tube to the second or

third Cathode jack when the Self Bias Resist-

ance control to set to any value above zero

ohms.

4. Rotate Short Test -Meter Reading switch to

"Short Test" position.

5. Select the required heater voltage by setting the

Heater Voltage Selector switch.

6. Insert tube in a socket corresponding to the pin

arrangement of the tube to be checked.

7. Place the Heater Supply toggle in the "On"

position. For those instruments connected for 230

volt line service see "Note" under paragraph headed

"HEATER SUPPLY TOGGLE SWITCH", on page 6.

8. Rotate Heater Voltage Adjuster to correct heater

voltage indicated on Heater Voltage meter.

.9. Rotate Element Short Test switch through its six

positions, stopping at each position to tap the tube.

10. A lighted neon lamp indicates a short in the

tube and no further tests should be conducted. A

lighted neon lamp on Position (1) indicates heater to

cathode leakage.

11. From time to time check neon lamp by placing

Lamp Test toggle in the upper position.

12. Set the Self Bias Resistance control to "Fixed

Bias" or zero ohms position. Note: If manufacturer's

rating specifically calls for self bias, set the control

to the proper value.

13. Rotate the Control Grid, Suppressor, Screen

and Plate adjusters to the extreme counter-clockwise

position.

14. Index Short Test -Meter Reading switch to

"Meter Reading" position.

IS. Place Plate Supply toggle switch in the "On"

position. .

16. Index Gill Factor switch to the "XIO" position.

The "Xl" position may be used for subminiature

tubes or low bias high mu tubes.

17. Index Gill Range switch to .a range consistent

with the expected mutual conductance. This is the

value listed by the manufacturer.

18. Index Element Current switch located below

the Gill Factor toggle switch to the "Plate" position.

19. Index Element Current Range switch located

below the Element Current meter to a position higher

than the expected plate current specified by manu-

facturer.

20. Rotate the Control Grid VoItage Adjuster un-

til the Control Grid Voltmeter indicates the value

specified by manufacturer. If over 10 volts, index

Control Grid switch to "50 Volts" position. Rotate to

extreme counter-clockwise position if Self Bias Re-

sistance control is set to any position other than

"Fixed Bias" or zero ohms.

21. Index the Element Voltage switch located un-

der the Element Voltage meter to the "Plate" position.

22. Advance the Plate Voltage Adjuster until the

Element Voltage meter indicates the plate potential

specified by manufacturer. If over 150 volts, index

Element Voltage Range switch to "300" volts position.

23. Index Element Voltage switch to "Screen" posi-

tion.

24. Advance Screen Grid Voltage Adjuster until

the Element Voltage meter indicates. the screen po-

tential specified by manufacturer. If over 150 volts,

index Element Voltage Range switch to "300" volts

position. Important Note: To prevent excessive screen

dissipation keep the plate potential the same as or

higher than the screen potential.

25. Recheck the plate, screen and control grid

voltages.

26. Plate, screen, suppressor or cathode currents

can be readily checked by rotating the Element

Current switch through its four positions noting the

current on the Element Current meter. It may be

necessary in doing this to change range on the Ele-

ment Current meter by rotating the Element Current

Range switch to one of the other positions.

27. If the tube is a filiamentary type, place the

Gill Factor toggle switch to the "x 1" position and

rotate the Hum Control to give minimum reading on

the fan-shaped Micromhos meter. Note: Serious errors

in mutual conductance readings can be expected if

this operation is not performed carefully on filament

types.

28. Pull the Microamperes toggle switch to the

"IS" position. Read microampere grid current di-

rectly on 15 scale. Refer to "Grid Current" under

Helpful Suggestions In Tube Testing on page 10.

-7-

29. Lift the Signal toggle switch to the "Cal." posi-

tion and rotate the Signal Amplitude Adjuster to

bring the Micromhos meter to exactly top mark.

30. Release Signal toggle switch.

31. To take the Gm reading, index the "Gill" tog-

gle switch to the "Reading" position and read the

Micromhos meter. Read the scale which corresponds

to the "Gm Range" switch setting and multiply by an

added factor of 10 if the "Gm Factor" switch is set to

the "XI0" position.

RECTIFIERTUBES:

32. Patch jumper leads as in steps "3 and 3-(B)."

If rectifier has double plates, patch one of the plates

to the Plate 1 jack and the other to Plate 2 jack.

33. Follow steps 4-10 inclusive. If double rectifier

plates have been patched as above, short check the

tube as in step 9 indexing the Plate toggle switch to

both "Plate I" and "Plate 2" positions.

34. Follow steps 12-15 inclusive.

35. Set G.mFactor switch to "xl0."

36. Turn "Signal Amplitude" adjuster to zero. Note:

If steps 35 and 36 are not followed, the emission

measurements will not be in error. However, follow-

ing these two steps reduces the Gm meter sensitivity.

37. Index Element Current switch to "Plate" posi-

tion.

38. Index Element Current Range switch to "120"

position.

39. Index Element Volts switch to "60."

40. Place Plate Supply toggle switch in the "On"

position.

41. Advance the Plate Voltage Adjuster carefully

until the Element Current meter indicates the current

listed in the Tube Data Chart suppli~d separately.

42. Reject the tube as bad if the plate voltage re-

quired to give the specified current is greater than

that shown in the Tube Data Chart su,Pplied sep-

arately.

43. Repeat the emission check on the second plate

in the same manner indexing the Plate 1-Plate 2

toggle to its other position.

44. If the two plates have materially different emis-

sion readings the tube should be rejected.

DIODEDETECTORS:

45. Patch jumpers as in step 32 and in addition

connect all other elements to cathode.

46. Follow steps 33-44 inclusive, except that in

step 38 the Element Current Range switch should be

indexed to the "3" position. Note: The 0.8 MA. limit at

10 volts maximum is usually considered satisfactory,

however, diodes normally pass considerably greater

current, some going as high as 2 or 3 MA.

CONVERTERAND MIXER-OSCILLATOR TYPES:

47. Patch the jumper leads in accordance with the

information in Tube Data Chart supplied separately.

A-The figure in parenthesis in the Tube Data

Charts refers to the pin connection. Example:

A type lA7-GT shows the figure (3) and (6) in

parenthesis in the column headed Plate Volts.

Hence pins 3 and 6 are connected together

and patched to the Plate 1 jack by means of

the jumper leads.

B-The letter (C) in parenthesis denotes Grid Cap.

Example: A type lA7-GT shows the figure (5)

and the letter (C) in parenthesis in the column

headed Control Grid VoHs. Hence pin 5 and

the tube's grid cap are connected together and

patched to the Control Grid jack by means of

the jumper leads.

48. Follow steps 4 through 31 omitting any re-

marks concerning "manufacturer's specifications"

and substituting "value specified in the Tube Data

Chart supplied separately.

49. Reject tube when the mutual conductance indi-

cation is below that value specified in the column

headed "Life End."

THYRATRONTUBES:

Thyratrons such as the 2050 and 2051 can be

checked in any type Model 686. The step-by-step

procedure given below is for the tube type 2050.

Checking other thyratrons involves the same pro-

cedure but it must be kept in mind that the tubes

should be of a type similar to the 2050 and 2051.

1. Make connections by use of patch cords in

accordance with the base diagram except that

a 100,000 ohm 1/2 watt resistor should be in-

serted in the grid lead to pin 5. Number 2 grid

should be connected directly to cathode.

2. Set G./IIFactor switch to "x 10".

3. Turn Signal Amplitude Adjuster to zero.

4. Set Element Current Range switch to "120" milli-

amperes.

5. Set Element Current switch to "Plate" posi-

tion.

6. Before inserting the tube apply the following

element voltages: . .

Filament _ _ _ ____ __ ___ _ _____ __ _. 6.3

Control Grid m___m m_. - 10volts d-c

Platem_m m m +212 volts d-c

7. Insert tube, readjust filament voltage and allow

to heat for at least 30 seconds.

8. Reduce the grid bias carefully until the thyra-

tron fires. .

-8-

9. LIMITS: If the tube fires between -3 and -1

volts on the grid, the tube is within manufac-

turing limits for both end point and variation

in new tubes.

Note: Once the tube fires, the grid loses control.

If it should be necessary to repeat the test, in-

crease the grid bias to -10 volts and decrease

the plate voltage to zero, and then reset it to

212 volts and repeat the above procedure.

The plate current for tubes 2050 and 2051

should not exceed 100 milliamperes and 75

milliamperes respectively. If it does, reduce

the plate voltage slightly to bring it within these

values.

Note also that when the thyratron fires the

plate voltage will drop to about 7-1/2 volts.

This is normal.

For low current thyratrons, a lower initial

plate voltage should be used, or a current lim-

iting resistor inserted in the plate patch cord

lead.

VOLTAGE REGULATORS:

The Model 686 can check voltage regulator tubes

similar to the 874 and VR-150-30 types. The pro-

cedure outlined below is for checking the 874 type.

1. Make connections by use of patch cords in

accordance with the base diagram. In the case

of the 874, Pin 1 is the cathode and Pin 3 is

the anode or plate.

2. Set Gm Factor switch to "x 10".

3. Turn "Signal Amplitude" control down to zero.

4. Set Element Current Range switch to "120" milli-

amperes.

5. Set Element Current switch to "Plate" posi-

tion.

6. Increase the plate voltage until conduction be-

gins. This should be at approximately 115 volts

for the type 874. (For other voltage regulators

refer to manufacturer's ratings).

7. To check the regulating characteristics vary

the plate voltage to produce anode currents be-

tween 10 and 50 milliamperes maximum. (For

other voltage regulators see manufacturer's

ratings.)

Note: The voltage applied to produce currents

between 10 and 50 milliamperes should be

within the limits indicated by the man ufac-

turer's ratings. In the case of the 874 the voltage

after conduction should not vary more than 7

when the current is varied from 10 to 50 milli-

amperes.

THEORYOF OPERATION

Essentially the Model 686 is a low impedance

power supply metered for potentials and currents

and provided with a means of introducing an a-c

signal into the grid bias line and measuring the a-c

component in the plate circuit.

The incoming a-c line energizes the d-c power

supply, the heater voltage supply and the a-c grid

signal transformer. The filament transformer control

is located in the primary leads thus providing proper

adjustment to take care of varying line voltage con-

ditions. A rotary switch selects secondary taps to

give the necessary heater voltages for all tubes.

The d-c power supply delivers potentials to the

plate, screen and suppressor controls. Following

these controls the potentials are metered and the cir-

cuit is so arranged that the element milliammeter

can be placed in each of the lines. A separate reg-

ulated power supply develops the voltage for grid

bias. This potential is likewise controlled by a poten-

tiometer. A separate voltmeter is used to measure

this potential and a microammeter is placed in the

circuit to detect the presence of grid current.

The a-c grid signal winding together with a signal

voltage divider is placed in series with the control

grid circuit to the tube. The proper signal voltages

are selected by the Gm Range switch which is con-

nected to the signal voltage divider. The injection

of the a-c grid signal into the grid bias circuit is a

function performed by the GmReading toggle switch.

The Gm meter is an amplifier-rectifier type a-c

instrument which is connected to the plate circuit for

measuring the a-c component of plate current.

The element potentials are fed to a multi-circuit

Short Test-Meter Reading switch. This switch pro-

vides the necessary circuit connections so that tube

elements may be either short checked by means of

the Element Short Test switch or energized by the

potentials from the power supply.

The a-c grid signal transformer has a separate

winding feeding a type 3A4 tube which supplies the

necessary d-c voltage for high sensitivity short check.

The Hum Control is simply a potentiometer placed

across the filament winding to provide the necessary

balance on the filament return when checking these

types. This is to prevent an additional signal (which

may either add to or subtract from the true grid sig-

nal) from appearing in the grid circuit causing a

modification in the a-c plate current component. The

Hurn Control is switched out of the circuit on heater

voltages above 10.

In checking a tube, the d-c potentials are applied

to the tube through the various controls. The a-c grid

signal is applied in series with the grid bias voltage,

and is measured by switching the Gmmeter to "Sig-

nal Cal." and held to a fixed value by rotating the

Signal Amplitude Adjuster. To measure a-c compo-

-9-

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